The increasingly tense situation in the Middle East such as Iraq and Iran, and the development of industrial infrastructure in developing countries such as China have intensified the competition for petroleum resources as materials or energy sources and consequently oil prices have been increased worldwide. Research has been then actively carried out for the development of alternative energy to oil and materials based on naturally-occurring substances as alternatives to petroleum materials.
Meanwhile, the industry addresses global warming by reducing carbon dioxide emission in product manufacturing. Research has been then actively carried out for the development of energy-saving apparatuses or processes and the development of recycling materials such as biodegradable materials or reliably safe products against environmental pollution.
The research of recycling materials actively focuses on biodegradable polyesters, in particular aliphatic polyesters, and there are a number of patent applications directed to such polymers. For example, Patent Documents 1 to 3 disclose thermoplastic biodegradable aliphatic copolyesters that contain an aliphatic dicarboxylic acid or ester thereof, an aliphatic or cycloaliphatic diol, and an unsaturated acid of natural origin or ester thereof. Patent Document 4 discloses a method of synthesizing biodegradable aliphatic polyesters from one or more aliphatic dicarboxylic acids or esters thereof and one or more linear or branched aliphatic glycols under catalysis of monobutylstannoic acid.
Further, Patent Document 5 discloses a biodegradable vegetable oil grease that contains a natural base oil or a polymerized triglyceride; at least one of an alkyl phenol, a benzotriazole and an aromatic amine; and a metal based material wherein the metal is an alkali or alkaline earth metal.
For the synthesis of polyesters containing ricinoleic acid from fats, Non-Patent Document 1 reports that ricinoleic acid lactone and lactide are mixed in a predetermined ratio at high temperatures with a Sn or Z compound as a catalyst to give a copolymer having a molecular weight of 5,000 to 16,200 and a melting point of 100 to 130° C. The use of such copolymers in DDS is studied. It is reported that such copolymers have lower crystallinity than polylactic acid and are hydrolyzed more easily. Non-Patent Document 2 reports that a random copolymer with a molecular weight of 6,000 to 14,000 is obtained by polycondensing ricinoleic acid and lactic acid at high temperatures in a predetermined ratio and vacuum treating the resultant polyester. In these non-patent documents, the polyesters are synthesized by metal-catalyzed ring-opening polymerization via lactone or by polycondensation at high temperatures under reduced pressure. The molecular weight of the thus-obtained polyesters containing ricinoleic acid is low. Further, these non-patent documents do not report properties or performances of the copolymer polyesters with lactic acid.
Patent Document 6 discloses a polyester compound wherein the molecular structure is formed from a hydroxy fatty acid and an aliphatic dicarboxylic acid and has an amino group at a molecular end. Patent Document 7 discloses a reactive biodegradable copolymer for medical material use wherein the ricinoleic acid content is controlled by regulating the polycondensation reaction between ricinoleic acid and lactic acid. These materials, however, are obtained by methods similar to the polymerization for the polyesters described in Non-Patent Documents 1 and 2.
Patent Document 8 discloses a crosslinked ricinoleic acid composition elastomer that is formed from castor oil or a ricinoleic acid derivative, an epoxidized oil and a polycarboxylic acid in the presence of a peroxide initiator. A sheet material of the elastomer is described to show good mechanical strength and elasticity and be resistant to abrasion and hydrolysis.
Enzyme-catalyzed polymerization has been reported as a method for the production of biodegradable polyesters other than by thermal polycondensation. In detail, a lipase that is a hydrolyzing enzyme is used to facilitate esterification in the equilibrium reaction. According to this method, polyesters are synthesized from fats or fatty acids using an enzyme lipase that is immobilized for efficient use of the lipase.
In this respect, Patent Document 9 discloses a process wherein a polyester is produced from ricinoleic acid with use of a lipase immobilized on a calcined zeolite carrier while controlling the water content in the carrier at not more than 800 mg per 1 g of the immobilized enzyme.
In the production of polyesters from ricinoleic acid according to the known literature as described above, the enzyme reaction in the polymerization has a lower optimum temperature than thermochemical reactions to enable energy-saving and does not involve harmful organic solvents or catalysts. Accordingly, this polyester synthesis method is favorable in terms of global warming and environmental pollution. In Examples of Patent Document 9, however, the dehydration-condensation rate for estolides is followed based on the neutralization value and no estolides showing a neutralization value of 30 or less are obtained in Examples. From the neutralization values described therein, the average molecular weights of the polyesters are estimated to be less than 3,000, that is, the polyesters have a relatively low molecular weight.    Patent Document 1: JP-A-2005-523355    Patent Document 2: JP-A-2005-523356    Patent Document 3: JP-A-2005-523357    Patent Document 4: JP-A-2002-539309    Patent Document 5: JP-A-H10-46180    Patent Document 6: JP-A-H05-125166    Patent Document 7: JP-A-2005-113001    Patent Document 8: JP-A-2006-516998    Patent Document 9: JP-A-H05-211878    Non-Patent Document 1: Biomacromolecules 2005, 6, 1679-1688    Non-Patent Document 2: Macromolecules 2005, 38, 5545-5553
The literature described above has not reached a technical level at which biodegradable polyesters that are aimed at oil independence and do not contribute to global warming or environmental pollution may be produced by industrial processes with economic advantages.
In the field of rubber products, which is an expected application of the present invention, natural rubbers that are generally-used nonpetroleum materials mainly have a polyisoprene skeleton in which isoprene molecules are cis-1,4-bonded. However, they contain large amounts of high-molecular weight gels and proteins, and this fact makes quality stabilization difficult. In addition, they are poor in plasticity and processability as they are, and the processing thereof entails mastication (molecular scission) and addition of various antioxidants to increase durability.
Various synthetic rubbers have been developed as materials supplementing these defects of the natural rubbers. However, such synthetic rubbers are of oil origin and are generally not biodegradable. These petroleum synthetic rubbers are diene rubbers such as butadiene rubbers (BR), isoprene rubbers (IR), chloroprene rubbers (CR), rubbers of isobutene and a little isoprene (HR), styrene/butadiene rubbers (SBR) and butadiene/acrylonitrile rubbers (NNR); and non-diene rubbers such as ethylene/propylene rubbers (EPM), copolymers of ethylene, propylene and a little non-conjugated diene (EPDM), Hypalon from reaction of polyethylene with sulfur dioxide and chlorine, urethane rubbers from addition polymerization of diol and diisocyanate, polysulfide rubbers from polycondensation of dichloroethane and sodium tetrasulfide, silicone rubbers from ring-opening polymerization of cyclic siloxanes, and fluororubbers from copolymerization of vinylidene fluoride and trifluorochloroethylene.
These synthetic rubbers have excellent weather resistance, oil resistance, solvent resistance, chemical resistance, abrasion resistance and heat resistance and are used in tires, belts, automotive parts and other various industrial parts. Further, the synthetic rubbers are used in a wider range of applications as complex materials with other characteristic materials.
The use of these synthetic rubbers has spread to various industrial fields. However, stable supply of oil that is the material of these synthetic rubbers has been threatened. The fact that the synthetic rubbers are petroleum polymers makes it very difficult to recycle the materials, and disposing the materials causes environmental pollution. Furthermore, it is needless to say that eliminating or saving organic solvents or thermal energy in the polymerization for these synthetic rubbers is advantageous. The polymers obtained according to the present invention may be used as alternatives to the conventional rubber materials.